Black chromium plating process

ABSTRACT

A process for producing a lustrous, deep black, corrosion resistant, adherent film upon a conductive base which comprises electrodepositing such film upon such base acting as a cathode, employing an electrolyte comprising: A. 20 - 35 percent by weight water; and B. 65 - 80 percent by weight of a mixture of: 1. 30 - 75 PERCENT BY WEIGHT CHROMIC ANHYDRIDE; AND 2. 25 - 70 PERCENT BY WEIGHT ACETIC ACID, WHILE MAINTAINING THE ELECTROLYTE AT A TEMPERATURE OF 60* TO 80* F. while passing a current therethrough at a current density of 125 to 1,000 amperes per square foot.

United States Patent 1 Oleson et al.

l 1March 13, 1973 BLACK CHROMIUM PLATING PROCESS Roger M. Woods, 2301 S.E.th Thomas Road, Charlotte, NC. 28210 [22] Filed: Nov. 30, 1970 [21] Appl. No.: 93,871

Related US. Application Data [63] Continuation-impart of Ser. No. 422,503, Dec. 31,

1964, abandoned.

[52] U.S. Cl ..204/51 [51] Int. Cl. ..C23b /06 [58'] Field of Search ..204/51, 56 R [56] References Cited UNITED STATES PATENTS 2,623,847 12/1952 Gilbert et al. ..204/51 5/1956 Blaine ..204/51 8/1944 Keuffeletal. ..204/51X Primary ExaminerG. L. Kaplan Attorney-Sherman 8L Shalloway [57] ABSTRACT A process for producing a lustrous, deep black, corrosion resistant, adherent film upon a conductive base which comprises electrodepositing 'such film upon such base acting as a cathode, employing an electrolyte comprising:

A. percent by weight water; and B. 80 percent by weight of a mixture of:

1. 30 percent by weight chromic anhydride;

and 2. 25 70 percent by weight acetic acid, while maintaining the electrolyte at a temperature of 601 to F. while passing a current therethrough at a current density of to 1,000 amperes per square foot.

4 Claims, No Drawings BLACK CHROMIUM PLATING PROCESS This application is a continuation-in-part of co-pending application Ser. No. 422,503 filed December 31, 1964, now abandoned.

The present invention relates to a process for producing a bright, lustrous black, corrosion resistant, strongly adherent electrodeposited film or coating of chromium and oxides of chromium upon a conductive base such as a metal base. More particularly, the present invention relates to such a process for producing an electrodeposited film or coating of chromium and oxides of chromium wherein the conductive base is employed as a cathode in an electroplating system utilizing as an electrolyte an aqueous solution of chromic anhydride and acetic acid.

In the past, various attempts have been made for producing an electrodeposited film or coating which is black in color by depositing chromium or chromium oxides from a plating bath containing chromic anhydride and acetic acid. For example, previous attempts at such electrodeposition of chromium, black in color, have involved concentration of acetic acid in the electrolyte bath of 5 to ml of glacial acetic acid per liter of solution. Utilizing such a bath, it was found necessary to operate at low temperatures i.e., in the neighborhood of 50F. and to deposit films thereon at very high cathode current densities i.e., about 1,000 to 2,000 amperes per square foot. With such low temperatures and high current densities it was found that the efficiency of the electrolyte bath was very low due in part to the necessity of cooling the bath by means of supplemental refrigeration equipment. In addition, it has been found that such processes could not be operated on a commercial scale with any advantage due to the high currents required. Moreover, the products produced by such previous processes have been somewhat inferior in quality since the films which have been electrodeposited have been poorly adherent to the base and of lighter color than desired i.e., not black in color. In addition to the foregoing, other attempts have been made to produce an electrodeposit which is black in color by employing a plating bath of glacial acetic acid and chromic anhydride, such as set forth for example in U.S. Pat. No. 2,623,847. In such an attempt however it was found necessary to maintain the temperature of the electrolyte bath in the range of from about 90F. to 150F. and the resulting plating was found to be non-lustrous, uneven and exhibited low light reflectivity as well as being grayish black in color. Accordingly, such proposal also has failed to produce a desirable product. p l

The foregoing disadvantages of previous proposals have been overcome in accordance with the present invention by which it has been found possible to produce a bright, lustrous black, corrosion resistant, strongly adherent electrodeposited chromium or chromium oxidefilm upon a conductive base. This is achieved in accordance with the present invention by utilizing such base as the cathode of an electroplating system in which the electrolyte comprises to 35% by weight water and 65 to 80 percent by weight of an admixture of (1) 30 to 75 percent by weight chromic anhydride preferably 40-80 percent chromic acid, and (2) to 70 percent by weight acetic acid preferably 20-60% acetic acid, the temperature of the electrolyte being maintained at a temperature of 60 to 80 F. While passing a current therethrough at a current density of 125 to 1,000 amperes per square foot.

Accordingly it is a principal object of the present invention to provide a process for the electrodeposition of a chromium or chromium oxide film, black in color, which process eliminates all of the various deficiencies and disadvantages of previous proposals.

It is a further object of the present invention to provide such a process for electrodepositing an extremely adherent, bright, lustrous black, chromium or chromium oxide film upon a conductive base, which film is highly resistant to corrosion, protecting the base material from corrosion.

It is still a further object of the present invention to provide such a process for the production of a lustrous, deep black, corrosion resistant, adherent chromium or chromium oxide film upon a conductive base, wherein such a film having an attractive and unique decorative appearance is achieved by electrodeposition of the chromium or chromium oxide from an electrolytic bath comprising water, chromic anhydride and acetic acid in specific proportions by weight, the conductive base acting as the cathode in the electroplating system, the electrolyte bath being maintained at a temperature of 60 to 80F. while passing a current therethrough at a current density of 125 to 1,000 amperes per square foot.

Still further objects and advantages of the novel process of the present invention will become more apparent from the following more detailed description thereof.

The foregoing objects and advantages of the present invention are achieved in accordance with a process whereby a conductive base material is electroplated in a system in which the base material is cathodic and in which the electrolyte comprises an aqueous solution of chromic anhydride (CrO and acetic acid. Although acetic acid is preferred, other homo-logues such as propionic acid, butyric acid, and valeric acid may be employed in accordance with the process of the present invention. I

In accordance with the present invention the electroplating system is one in which the electrolyte comprises:

A. 20 35 percent by weight water; and

B. 65 80 percent by weight of an admixture of l. 30 percent, preferably 40-80 percent by weight chromic anhydride; and 2. 25 70 percent, preferably 20-60 percent by weight acetic acid. As indicated previously, such electroplating system is such that the conductive base is employed as the cathode. In addition it is essential in accordance with the present invention that the electrolyte be maintained at a temperature of 60 to F. during density of to 1,000 amperes per square foot.

In accordance with the present invention, the electrolyte bath is preferably produced by adding glacial acetic acid which has been diluted with water to approximately an 80 percent acetic acid solution to chromic acid flakes in a suitable mixing vessel. After addition of the diluted glacial acetic acid to the chromic acid flakes, additional water is added to produce the electrolyte solution having a concentration within the limits set forth above. A preferable electrolyte solution in accordance with the present invention comprises 44 parts by weight (34.6 percent) of chromic anhydride, 48 parts by weight (37.8 percent) of glacial acetic acid, and 35 parts by weight (27.6 percent) water.

In accordance with the present invention it has been determined that the presence of sulfate ions in the electrolyte bath is deleterious to the formation of a desirable lustrous black, chromium or chromium oxide coating. Accordingly, in order to remove the sulfate ions, the sulfate is precipitated out of the electrolyte bath through the addition of a small amount of a barium compound, preferably barium acetate added to the aqueous solution of chromic anhydride and acetic acid.

The aqueous solution which is prepared is mixed vigorously and after the solution is suitably mixed, the bath is broken in or conditioned at a temperature of at least ll5F. for a minimum of 2 hours, typically at a voltage of 6 to 12 volts. The breaking in of the bath effects the disassociation of the compounds in the bath enabling the bath to operate satisfactorily during subsequent electrodeposition. The breaking in or conditioning of the bath may be accomplished by exposing the bath to ordinary plating conditions wherein a dummy load is placed in the bath and is rendered cathodic during the breaking in or conditioning period.

After the breaking in or conditioning of the electrolyte bath, the object or part to be plated is then immersed in the bath and is made the cathode of the electrical system. While the anode employed in the electrical system can be composed of various materials it has been found preferably in accordance with the present invention to utilize an anode of carburized steel. In this regard, although stainless steel, carbon and lead anodes may be employed in the process of the present invention, such anode materials are employed with slightly less satisfactory results.

In connection with the above it should be noted that an unprotected lead tank should not be used to hold the electrolyte bath. It has been found desirable in accordance with the present invention to use a tank lined with polyvinyl chloride or other suitable synthetic inert materials. In addition the plating tank should be provided with an exhaust system to carry away fumes of acetic acid and chromic acid.

While the process of the present invention can be carried out within the concentration, temperature and current density limits set forth above, it is preferred in accordance with the present invention to operate the electroplating bath at a temperature of approximately 70 to 71F. Moreover, it is preferred in accordance with the present invention that the plating be carried out at a cathode current density of approximately 300 to 400 amperes per square foot; however, satisfactory plating can be effected at a current density as low as 125 amperes per square foot or even lower where desired for specific purposes or, if practical, as high as 1,000 amperes per square foot. Under such conditions, a very short plating time, even as short as 30 seconds, will produce a deep black film of uniform color consistency.

In this regard, it has been discovered in accordance with the present invention that the electroplating bath of the present invention can be successfully operated at extremely high current densities, for example on the order of 1,000 amperes per square foot, without the deleterious burning of the part being plated. With conventional electroplating baths the burning of the part being plated often occurs, due to an increased current density, causing the part being plated to be rejected and thus decreasing producing economics.

In accordance with the present invention, after the plating process, the plated article is removed from the bath, rinsed in a conventional manner and dried by an air blast or wiping with an absorbent material. The resulting plating or finish is highly lustrous and there is absolutely no need for any subsequent buffing or polishing operations. As indicated previously, the process of the present invention allows for the production of a lutrous,deep black, corrosion resistant, adherent chromium or chromium oxide film upon any conductive base.

With regard to preferred procedures in accordance with the present invention, it has been found desirable to maintain the anode area as large as possible with respect to the area ofa cathode in order to maintain the anode current density at relatively low value, thereby avoiding coating of the anode with a film. While the nature of the film is not fully understood nor are the results attributed thereto, the coating of the anode with such a film tends to increase the resistance of the system. Accordingly the anode area should be at least as great as and preferably three or four times as great as the cathode area for the most efficient operation of the process of the present invention.

Additionally, while the exact composition of the film which results from the process of the present invention is not yet fully determined, it is believed that the film contains a high proportion of metallic chromium and a low proportion of chromium oxide. In addition, the exact composition of the resulting film differs according to the use of different conditions within the parameters of the present invention.

The plating bath may be replenished by the addition of chroinic anhydride and acetic acid as they are consumed in the plating operation. In actual practice, this may be done by adding the two compounds in the ratio of approximately 111 by weight during the course of the plating operation. A sufficiently accurate control for production use may be achieved by specific gravity measurements of the plating bath through the use of a hydrometer or similar means. In the event the plating becomes sluggish and exhibits poor throwing power in use, electrolytic conversion of the trivalent chromium, readjustment of the acetic acid content and addition of water will typically restore the bath to proper operating conditions.

The process of the present invention will now be illustrated by reference to the following example. It is to be understood that such example is presented for purposes of illustration only and is in no way to be deemed as limitive of the present invention.

EXAMPLE An electroplating bath was prepared by placing 44 ounces of chromic acid flake CrO into a mixing vessel. Glacial acetic acid which has been diluted by water to produce a resulting acetic acid solution was added to the chromic acid flake in an amount of 48 ounces of the diluted acetic acid. Thereafter, 35 ounces of water were added to the chromic acid and diluted acetic acid solution. Finally, one ounce of barium acetate was added to the above solution.

The solution resulting from the mixture of the foregoing materials was mixed vigorously and thereafter utilized in an electrical system as the electrolyte. The electrolyte bath was broken in or conditioned at a temperature of at least 115F. for a period of at least two hours at voltage of approximately 6 to 12 volts. Such breaking in of the bath effects a disassociation of the compounds thereby allowing the bath to operate satisfactorily in a subsequent electrodeposition process.

A conductive base metal, i.e., steel, was immersed in the bath and made the cathode of the electrical system. The anode of the electrical system was made of carburized steel with the anode area being approximately three to four times as great as the cathode area.

In the electrodeposition process, the electrolyte bath was maintained at a temperature of approximately 70 to 71F. and the plating of the chromium film was conducted at a cathode current density of approximately 300 to 400 amperes per square foot. By operating under such conditions it was found possible to produce a lustrous, deep black, corrosion resistant, adherent film upon the conductive base.

It can be seen from the foregoing that through the use of a specifically defined electrolyte solution containing specific amounts of water, chromic anhydride and acetic acid and through the use of a specific temperature and current density, it has been possible to provide a process and product not heretofore possible with previous prior art proposals. Accordingly, pursuant to the present invention it has been found possible for the first time to produce a bright, lustrousblack, corrosion resistant, strongly adherent electrodeposited film or coating of chromium and chromium oxides upon a conductive base i.e., a metal base.

While the present invention has been described primarily with regard to the foregoing exemplification it should be understood that the present invention is not in any way to be deemed as limited thereto but, rather, must be construed as broadly as all or any equivalents thereof.

What is claimed is: 1. A process for the production of a lustrous, deep black, corrosion resistant, adherent film upon a con- 5 ductive base which consists essentially of:

providing said base as the cathode of an electroplating system in which the electrolyte consists essentially of: A. 20-35 percent by weight water; and B 65-80 percent by weight of an admixture of:

1. 30-75 percent by'weight chromic anhydride;

and 2. 25-70 percent by weight acetic acid; and maintaining said electrolyte at a temperature of 60 to 80 F. while passing a current therethrough at a current density of 125 to 1,000 amperes per square foot measured at the cathode.

2. A process for the production of a lustrous, deep black, corrosion resistant, adherent film upon a conductive metal base which consists essentially of:

providing said base as the cathode of an electroplating system in which the electrolyte consists essentially of:

A. 20-35 percent by weight water; and

B. 65-80 percent by weight of an admixture of:

1. 40-80 percent by weight chromic anhydride;

and

2. 20-60 percent by weight acetic acid; and maintaining said electrolyte at a temperature of 70 to 71 F. while passing a current therethrough at a current density of 300 to 400 amperes per square foot measured at the cathode.

3. The process of claim 1 wherein said electrolyte consists essentially of:

27.6 percent of weight water: 34.6 percent weight chromic anhydride; and 37.8 percent by weight acetic acid. 4. The process of claim 2 wherein saidelectrolyte 40 consists essentially of:

27.6% by weight water; 34.6% by weight chromic anhydride; and 37.8% by weight acetic acid. 

1. A process for the production of a lustrous, deep black, corrosion resistant, adherent film upon a conductive base which consists essentially of: providing said base as the cathode of an electroplating system in which the electrolyte consists essentially of: A. 20-35 percent by weight water; and B. 65-80 percent by weight of an admixture of:
 1. 30-75 percent by weight chromic anhydride; and
 1. 40-80 percent by weight chromic anhydride; and
 2. 20-60 percent by weight acetic acid; and maintaining said electrolyte at a temperature of 70* to 71* F. while passing a current therethrough at a current density of 300 to 400 amperes per square foot measured at the cathode.
 2. 25-70 percent by weight acetic acid; and maintaining said electrolyte at a temperature of 60* to 80* F. while passing a current therethrough at a current density of 125 to 1,000 amperes per square foot measured at the cathode.
 2. A process for the production of a lustrous, deep black, corrosion resistant, adherent film upon a conductive metal base which consists essentially of: providing said base as the cathode of an electroplating system in which the electrolyte consists essentially of: A. 20-35 percent by weight water; and B. 65-80 percent by weight of an admixture of:
 3. The process of claim 1 wherein said electrolyte consists essentially of: 27.6 percent of weight water: 34.6 percent weight chromic anhydride; and 37.8 percent by weight acetic acid. 